Protein engineering is a powerful tool for modification of the structural catalytic and binding properties of natural proteins and for the de novo design of artificial proteins. Protein engineering relies on an efficient recognition mechanism for incorporating mutant amino acids in the desired protein sequences. Though this process has been very useful for designing new macromolecules with precise control of composition and architecture, a major limitation is that the mutagenesis is restricted to the 20 naturally occurring amino acids. However, it is becoming increasingly clear that incorporation of non-natural amino acids can extend the scope and impact of protein engineering methods.1 Thus, for many applications of designed macromolecules, it would be desirable to develop methods for incorporating amino acids that have novel chemical functionality not possessed by the 20 amino acids commonly found in naturally occuring proteins. That is, ideally, one would like to tailor changes in a protein (the size, acidity, nucleophilicity, hydrogen-bonding or hydrophobic properties, etc. of amino acids) to fulfill a specific structural or functional property of interest. The ability to incorporate such amino acid analogs into proteins would greatly expand our ability to rationally and systematically manipulate the structures of proteins, both to probe protein function and create proteins with new properties. For example, the ability to synthesize large quantities of proteins containing heavy atoms would facilitate protein structure determination, and the ability to site specifically substitute fluorophores or photo-cleavable groups into proteins in living cells would provide powerful tools for studying protein functions in vivo. One might also be able to enhance the properties of proteins by providing building blocks with new functional groups, such as an amino acid containing a keto-group.
Incorporation of novel amino acids in macromolecules has been successful to an extent. Biosynthetic assimilation of non-canonical amino acids into proteins has been achieved largely by exploiting the capacity of the wild type synthesis apparatus to utilize analogs of naturally occurring amino acids (Budisa 1995, Eur. J. Biochem 230: 788–796; Deming 1997, J. Macromol. Sci. Pure Appl. Chem A34; 2143–2150; Duewel 1997, Biochemistry 36: 3404–3416; van Hest and Tirrell 1998, FEBS Lett 428(1–2): 68–70; Sharma et al., 2000, FEBS Lett 467(1): 37–40). Nevertheless, the number of amino acids shown conclusively to exhibit translational activity in vivo is small, and the chemical functionality that has been accessed by this method remains modest. In designing macromolecules with desired properties, this poses a limitation since such designs may require incorporation of complex analogs that differ significantly from the natural substrates in terms of both size and chemical properties and hence, are unable to circumvent the specificity of the synthetases. Thus, to further expand the range of non-natural amino acids that can be incorporated, there is a need to manipulate the activity of the aminoacyl tRNA synthetases (AARS).
Protein translation from an mRNA template is carried out by ribosomes. During the translation process, each tRNA is matched with its amino acid long before it reaches the ribosome. The match is made by a collection of enzymes known as the aminoacyl-tRNA synthetases (AARS). These enzymes charge each tRNA with the proper amino acid, thus allowing each tRNA to make the proper translation from the genetic code of DNA (and the mRNA transcribed from the DNA) into the amino acid code of proteins.
Most cells make twenty different aminoacyl-tRNA synthetases, one for each type of amino acid. These twenty enzymes are each optimized for function with its own particular amino acid and the set of tRNA molecules appropriate to that amino acid. Aminoacyl-tRNA synthetases must perform their tasks with high accuracy. Many of these enzymes recognize their tRNA molecules using the anticodon. These enzymes make about one mistake in 10,000. For most amino acids, this level of accuracy is not too difficult to achieve, since most of the amino acids are quite different from one another.